1. Field of the Invention
This invention relates to non resonant antennas and more particularly broadband antennas. Exceptionally wide frequency bandwidth antennas may include matching a 50 ohm coaxial cable transmission line with a typical Voltage Standing Wave Ratio (VSWR) of 3:1, but may be made lower if required with a slight loss in efficiency. The invention may be used in very low frequency (VLF), high frequency (HF), very high frequency (VHF), and ultra high frequency (UHF) antenna system applications. This invention can also replace existing monopole, dipole, long wire and low/zero profile ground or mast mounted antennas without special mechanical mounting requirements or major changes in existing antenna installations.
2. Description of the Related Technology
Radio frequency antennas may be grouped into two categories: resonant, and non-resonant/terminated. The quarter wave monopole or half-wave dipole are typical examples of resonant antennas while a rhombic antenna is an example of non resonant terminated type. In a resonant antenna, radio frequency current flows from the point the radio frequency power source connects to the antenna then goes to the opposite end of the antenna where the current flow subsequently reflects back and forth. In a non resonant terminated antenna, radio frequency current flow travels only once from the power source to a termination load at the end of the non resonant antenna. The termination load is usually a resistor equal to the impedance of the antenna viewed as a transmission line. As the radio frequency wave travels along the wire antenna element the antenna impedance undergoes a distributed rise. The resistor value is chosen such that it matches the wire distributed impedance at the point of its insertion into the wire. The effect of this is that the remaining (un radiated) forward traveling wave does not experience any further variation in impedance. Upon reflection, however, the returning wave is presented with a point impedance differing from the impedance of the wire. This reflected energy is lost partly by radiation, but more significantly, loss is due to absorption in the resistive element.
Resonant antennas are more efficient for a given length of wire but are restricted to narrow bandwidths. Non resonant antennas function over a very wide range of frequencies and the radiation pattern is more directive when the length of the antenna is multiple wavelengths of the radio frequency.
Most present day users of antennas for communication purposes desire an antenna that can operate on multiple frequencies without requiring antenna tuning or matching devices. For example, present HF antennas must operate over the typical range of from 1.5 MHz to 30 MHz. This wide frequency range requires the use of an antenna tuner which limits the useful bandwidth of the HF antenna to 10 KHz or less at the lower frequencies. This narrow bandwidth precludes the use of modern spread spectrum and frequency agile communications techniques, and causes ringing and undesirable distortion in high speed wide bandwidth data transmission. In addition, the antenna coupler, being a lossy capacitor inductor network, can result in power losses comprising a major portion of the available transmitter power at the lower frequencies.
Each time a new frequency is selected the antenna tuner must retune the entire antenna system, taking from 2 to 90 seconds to accomplish. Use of voltage tuneable networks, typically using diodes and varactors as tuning devices, have been attempted in order to decrease the tuning time. These voltage tuneable networks, however, have proven to be expensive and limited to low transmitting power. In addition, severe harmonic and intermodulation distortion results from the nonlinear radio frequency characteristics of the diode and varactor components.
Frequency agile communication techniques requiring rapid and wide frequency excursions are impossible when a tuner is required to resonate the antenna. In addition, multiple transmitters, widely spaced in frequency, cannot use a single resonant antenna because of its narrow bandwidth. Thus, simultaneous multiple transmitter operation requires multiple resonant antennas. Broadband non resonant antennas do not have these limitations.
Typically, aircraft, low/zero profile ground or mast mounted, and shipboard HF antenna installations restrict the actual length of the antenna. Typically, the maximum allowable length of a shipboard whip HF antenna is 35 feet and an aircraft wire HF antenna is 64 feet. Close proximity to metal structures or other antennas cause detuning of a resonant antenna with subsequent radiation pattern discontinuity. In addition, resonant antennas have radiation pattern nulls caused by antenna lobe pattern activity at the resonant length of the wire. Broadband HF antennas do not have as significant a problem with radiation pattern discontinuities due to close proximity to metal structures or other antennas.
It is known in the art as taught by Altshuler, "Traveling Wave Linear Antenna," IRE, Jul. 1961, that the use of a dissipative load resistor inserted in a monopole antenna will absorb the reflected wave. The load resistor is introduced at some distance from the bottom of the monopole. This distance, typically, is two thirds of the length of the antenna as measured from the feed point of the monopole. Using this type of configuration, only two thirds of the antenna height effectively radiates, the upper one third of the antenna contributes little, if any, radiation of radio frequency energy.
As the radio wave travels along the antenna the impedance undergoes a distributed rise as it does for any antenna. The insertion resistor value is chosen to match the antenna distributed impedance at the point of its connection into the radiating element. The effect of this is that the remaining (unradiated) forward traveling wave does not experience any further impedance variation. Upon reflection, however, the returning wave is presented with a point impedance differing from that of the antenna, causing additional reflection of the wave. Thus, the remaining energy is lost partly by radiation, but more significantly loss is due to absorption in the resistive insertion element.